Image tube

ABSTRACT

An optical image is projected onto a planar surface of a photocathode that derives an electron beam replica of the image. A target electrode displaced relative to the photocathode so that it does not obstruct the optical image includes a planar surface for receiving and deriving an accurate replica of the electron beam image. The two planar surfaces are parallel. The electron beam image is focused on the target electrode by providing throughout a region that extends between the planar surfaces of the photocathode and receiving electrode, constant, homogeneous d.c. electric and magnetic fields that are canted relative to each other. The electric field extends in a direction perpendicular to the planar surfaces while the magnetic field extends along a straight line that intersects the photocathode and target electrode at an acute angle.

United States Patent [1 1 Low et a1.

[ IMAGE TUBE [76] Inventors: George M. Low, Administrator of theNational Aeronautics and Space Administration with respect to aninvention of; Kenneth L. Hallam, Washington, DC; Charles Bruce Johnson,Southfield, Mich.

[22] Filed: Oct. 16, 1972 [211 Appl. No.: 298,157

[52] U.S.Cl ..315/l0,3l5/ll,3l5/l2 [51] Int. Cl. H01] 31/26 [58] Field01 Search 315/10, 11, 12; 313/65 R [56] References Cited UNITED STATESPATENTS 7/1941 Flory et al 313/65 R X 10/1941 Bedford 313/65 R X3,478,216 11/1969 Carruthers 313/65 R X 3,662,104 5/1972 Nordseth eta1.. 315/10 X 2,731,560 1/1956 Krawinkel 315/12 X 2,788,452 4/1957Sternglass 315/11 X 2,853,643 9/1958 Theile 315/11 3,396,305 8/1968Buddecke et a1. 315/12 3.622.104 5/1972 Ngrglseth et a1 ,.3l5/l0 X \NPUTWlNDOW [451 Apr. 23, 1974 Primary Examiner-Leland A. Sebastian AssistantExaminer-P. A. Nelson Attorney, Agent, or FirmR. F. Kampf; John R.Manning [57] ABSTRACT An optical image is projected onto a planarsurface of a photocathode that derives an electron beam replica of theimage. A target electrode displaced relative to the photocathode so thatit does not obstruct the optical image includes a planar surface forreceiving and deriving an accurate replica of the electron beam image.The two planar surfaces are parallel. The electron beam image is focusedon the target electrode by providing throughout a region that extendsbetween the planar surfaces of the photocathode and receiving electrode,constant, homogeneous d.c. electric and magnetic fields that are cantedrelative to each other. The electric field extends in a directionperpendicular to the planar surfaces while the magnetic field extendsalong a straight line that intersects the photocathode and targetelectrode at an acute angle.

7 Claims, 3 Drawing Figures IMAGE TUBE ORIGIN OF THE INVENTION Theinvention described herein was made in the performance of work under aNASA contract and is subject to the provisions of Section 305 of theNational Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat.435; 42 U.S.C. 2457).

FIELD OF INVENTION BACKGROUND OF THE INVENTION For many applications, itis desirable to convert an optical image to an electron beam image suchthat the electron beam image is derived from the same side of aphotocathode as isirradiated by the optical image, and the electron beamimage is a reflected replica of the optical image. The optical image maycontain energy in the infrared, visible or ultraviolet spectrums. Imageconverters of this type, herein referred to as reflective imageconverters are particularly advantageous with opaque photocathodes.Opaque photocathodes are advantageous over the now generally utilizedsemitransparent photocathodes because they have increased efficiency,are easier to fabricate, utilize compounds from Group lIl-V that areresponsive to wavelengths between ultraviolet (approximately 100nanometers) and the near-infrared (approximately 1,200 nanometers), andare highly sensitive to vacuum ultraviolet range wavelengths. Reflectiveimage converters also enable a semi-transparent photocathode to beresponsive to optical images directed onto the photocathode fromopposite directions, whereby a first optical image can be focused toimpinge directly on an electron emitting surface of the photocathode anda second optical image can, if desired, be simultaneously focused toimpinge on the electron emitting surface via a transmission path throughthe photocathode.

Practical prior art reflective image converters have generally employedoptical reflectors to direct the optical image onto the photocathode.The photocathode or a target electrode for receiving the electron beamimage of the photocathode is usually in the path of the optical image sothat a portion of the optical image is obstructed thereby and a blankarea in the image occurs, usually in the image center.

Although reflective image converters have been proposed wherein aphotocathode does not obstruct the optical image, apparently none ofthese converters has been adopted. Analysis indicates that these priorart proposals result in target electron images that do not appear to beacceptable, reproducible replicas of the optical image. In general, theresolution and/or magnification of the electron beam image striking thetarget electrode would appear to vary from one area to another on thetarget electrode or be unpredictable from one unit to another.

.In one proposed type of prior art converter, an electron beam isderived from an opaque, planar photocathode in response to an impingingoptical image. The derived electron beam is accelerated and focused bycolinear, constant and homogeneous electric and magnetic fields. Afterthe electron beam has passed out of the region where the fields areconstant and homogeneous, it is bent by a deflecting magnet system to atarget electrode that is out of the line of sight of the optical image.Because the electron beam is bent in a region where the focus andaccelerating fields are no longer homogeneous and constant, the electronbeam has a tendency to diverge and an accurate replica of the originallyderived beam would not appear to be provided on the target electrode.

In a further proposed prior art system, a planar, opaque photocathoderesponds to-an optical image to derive an electron beam that isaccelerated by a constant d.c. electric field to a planar target whichis positioned so it does not obstruct the optical axis, a resultachieved by canting the accelerating electric field relative to theoptical image axis. A relatively thin solenoid provides a magnetic fieldhaving relatively short, straight lines of flux in a portion of theregion between the photocathode and target electrode. The angularposition of the solenoid axis relative to the electric field axisbetween the photocathode and target electrode is variable, as are themagnitudes of the magnetic field produced by the solenoid and theelectric field between the photocathode and receiving electrode. Theposition of the solenoid, and strengths of the magnetic and electricfields are varied depending upon the geometry of the situation, andapparently cannot be easily and predictably determined before aparticular unit is built.

BRIEF DESCRIPTION OF THE INVENTION In accordance with the presentinvention, the aforementioned problems of the prior art reflective imageconverters are avoided by utilizing an electron lens system wherein theentire region between an electron emissive surface of a photocathode andan electron receiving surface of a target electrode responsive toelectrons emitted from the photocathode includes constant, homogeneouselectric and magnetic fields having longitudinal vectors that are cantedrelative to each other. The longitudinal magnetic field vector extendsalong an axis between the photocathodeand target electrode. The magneticfield axis intercepts the emissive and receiving surfaces of thephotocathode and target at an acute angle greater than zero so that thetarget can be laterally displaced relative to the photocathode and thereis no obstruction of the optical image. The electric field longitudinalvector extends in a direction along an axis that extends at right anglesbetween the parallel, planar emitting and receiving sur faces of thephotocathode and target electrode. The electron lens system enables theemissive and receiving surfaces of the photocathode and target to beflat, a feature which enables the device of the present invention to beeasily fabricated.

The separation between the planar surfaces of the photocathode andtarget electrode is such that the electron image of the photocathode isfocused on the target electrode. In particular, the separation equalsthe distance whereby one cyclotron oscillation period exists between thephotocathode and target electrode. The homogeneous, uniform, cantedmagnetic and electric fields combine to provide constant radial velocityin planes parallel to the parallel planar surfaces and perpendicular tothe plane containing the two fields. The electric field providesconstant acceleration for electrons as they travel between thephotocathode and target electrode along and parallel to the magneticfield axis, and cyclotron motion about the magnetic field axis. Thesethree separate conditions of motion, together with the defined spacingbetween the photocathode and receiving electrode, result in an accuratereplica of the electron image derived from the photocathode to be formedover a substantial area of the receiving electrode.

It is, accordingly, an object of the present invention to provide a newand improved optical to electron image converter.

A further object of the present invention is to provide a new andimproved reflective image converter wherein an optical image impingesunobstructed on a photocathode.

A further object of the present invention is to provide a new andimproved reflective image converter wherein an accurate electron beamreplica of an optical image is predictably provided.

A further object of the invention is to provide a new and improveddevice for forming an electron beam image that is a composite of opticalimages received by a semi-transparent photocathode from oppositedirections.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of several specific embodiments thereof,especially when taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a planar, schematic view ofone embodiment of the invention utilizing an opaque photocathode;

FIG. 2 is a front view of the embodiment illustrated in FIG. 1; and

FIG. 3 is a plan view of a second embodiment of the invention, utilizinga semi-transparent photocathode.

DETAILED DESCRIPTION OF THE DRAWING Reference is now made to FIGS. 1 and2 of the drawing wherein there is illustrated an embodiment of theinvention wherein a vacuum electron tube 11 includes an opaquephotocathode electrode 12 at one end and an electron beam targetelectrode 13 at its other end. Target 13 may be a phosphorous screen toprovide direct optical readout of the electron beam image derived fromphotocathode 12 or a storage electrode that is readout by an electronbeam, such as disclosed in FIG. 5 of Bedford, US. Pat. No. 2,258,728.Electrodes 12 and 13 include planar, parallel facing surfaces 9 and 10that are in line of sight relationship with each other for respectivelyemitting and receiving an electron beam. The planar faces of electrodes12 and 13 are laterally displaced relative to each other, whereby anoptical image having optical axis 14 may be focused by lens 15 andpropagated through optically transparent window 16 of tube 11 onto theemitting face 9 of the photocathode, without being obstructed by target13. In response to the optical image focused on surface 9, thephotocathode 12 emits an electron beam that is focused by theelectron-lens of the present invention onto the facing surface 10 of thephosphorous screen 13.

The electron lens includes means for establishing a constant, d.c.homogeneous electric field having a longitudinal vector in tube 11 thatextends along and parallel to optical axis 14 and means for providing aconstant, d.c. homogeneous magnetic field having a longitudinal vectorthat extends between photocathode 12 and screen 13 along and parallel toaxis 17 that is displaced or canted by an angle 0 from the optical axis.

The constant, homogeneous electric field is preferably established byproviding a plurality of spaced, parallel, annular, metal rings 18 onthe inner circumferential wall of dielectric envelope 19 of tube 11.Preferably, adjacent ones of rings 18 are equidistant from each otherand the planar surfaces 9 and 10 of electrodes 12 and 13. Negative andpositive terminals of d.c. power supply 21 are respectively connected toelectrodes 12 and 13 and a different intermediate potential isestablished on each of electrodes 18 by taps 22 of resistive voltagedivider 23. The voltage at each of taps 22 is proportional to thedistance of the particular electrode 18 to which it is connected betweenelectrodes 12 and 13. To provide uniform, constant electric fields inthe region between electrode 12 and its adjacent electrode 18 and in theregion between electrode 13 and its adjacent electrode 18 in the regionthrough which the electron beam propagates, electrodes 12 and 13 aremade sufficiently large so there is no substantial fringing field in theregion traversed by the electron beam. In the alternative, the end wallsof envelope 19 can be provided with metallic coatings which areconnected to supply 21. In such a configuration, window 16 is anoptically transparent, metal coating or a metal mesh to establishoptically transparent equipotential planes at either end of envelope 19.

To attain the uniform, homogeneous magnetic field along axis 17throughout the region between the planar, parallel faces of electrodes12 and 13, a relatively long solenoid 23 having a longitudinal axisparallel with axis 17 and a length that extends beyond the planarelectron emitting and receiving faces 9 and 10 is located outside ofenvelope l9. Opposite ends of the solenoid 23 are connected to asuitable d.c. source 24 to establish the d.c. constant, homogeneousmagnetic field along and parallel to axis 17 throughout the volume oftube 11. It is to be understood that a similarly dimensioned andpositioned permanent magnet may be substituted for the solenoid and itsd.c. excitation or that a permanent magnet and an electric magnet may beused in combination.

In operation, the optical image on axis 14 is focused by lens 15 on theplanar surface of photocathode 12 which emits an electron beam that isaccelerated by the homogeneous constant electric field and focused bythe magnetic field produced by solenoid 23 along axis 17. The electronbeam is uniformly accelerated along and parallel to axis 17 because ofthe combined actions of the homogeneous, constant electric and magneticfields. The magnetic field also imparts cyclotron motion to electrons ofthe beam in a direction at right angles to axis 17. To provide thecorrect focus for the electron beam image at one or more integralcyclotron periods (N) or nodes of electron beam rotation (360 X N ofelectron beam rotation due to cyclotron action), the separation alongaxis 14 between the planar surfaces of target 13 and photocathode 12, L,and the cant angle 0, between the electric and magnetic field axes 14and 17 are related by:

L 1r NcosO/B V ZV/n where:

1 charge to mass ratio of an electron,

V d.c. potential difference between electrodes 12 and 13, and

B magnitude of magnetic flux density produced by solenoid 23.

The electric and magnetic fields combine to provide constant, radialvelocity in all planes parallel to surfaces 9 and 10 perpendicular tothe plane common to axes 1,4 and 17. It can be shown mathematically thatthe electron beam has a constant velocity, in planes perpendicular tothe plane containing axes 14 and 17 and parallel to the planar surfaces9 and 10 in accordance with:

(2). where:

= electric field vector parallel to axis 14, and

B magnetic field vector parallel to axis 17.

It can also be shown that there is a constant acceleration of anelectron in a direction parallel to axis 17 in accordance with:

T) i/ 7 E1 v acceleration vector of an electron in a direction ,parallelto axis 17, and E, electric field vector in a direction parallel to axis17. The cyclotron motion ofan electron about axis 17 can bemathematically expressed as:

8 where:

57,, cyclotron angular frequency vector of an orbiting electron.

The center of the optical image is shifted by the electron lens of thepresent invention from a position coincident with optical axis 14 tooutput optical axis 25 in the plane containing axes 14 and 17, i.e.,horizontally, by the distance D L tan (5). The output optical axis isshifted vertically, as illustrated in FIG. 3 in the electron receivingplane of target 13 by a distance 0', where 0'=[ IE x B ]3 l](21r/1 B)where:

total electric field vector, and

B total magnetic field vector.

In response to photocathode 12 being irradiated by monochromatic opticalenergy, the electrons derived from the photocathode have an emissionenergy spread which is small compared to their final energy when theystrike the target electrode 13; the emission spread is so small that theenergy can be considered quasi-monoenergetic. The quasi-monoenergeticelectrons provide a substantial replica on target 13 of the electronbeam image derived from photocathode 12 regardless of the angle 0between axes l4 and 17.

However, in response to photocathode 12 being irradiated by a relativelywide spectrum of optical energy, whereby electrons with significantlydiffering initial velocities are emitted by the photocathode, theelectrons impinging on target 13 produce a circle of confusion having afinite radius. It has been found that the circle of confusion has anegligible radius for a relatively wide spectrum of incident opticalenergy if the angle between axes 14 and 17 is limited so that it is nomore than approximately thirty degrees.

In one experimental tube fabricated in accordance with the presentinvention, the separation between the planar, parallel electron emittingand receiving surfaces of electrodes 12 and 13, L, was thirteencentimeters and a voltage of 9.0 KV was applied between the electrodes.A vertical deflection 0', of 4.0 millimeters was observed and thehorizontal shift from axis 14 to axis 25, for a deflection angle of 015, was 3.0 centimeters.

While the present invention is ideally suited for use in conjunctionwith opaque photocathodes, the principles of the invention are equallyapplicable to a semitransparent photocathode. In such a configuration,the optical image is directed at the photocathode from a directionbehind the electron emissive surface thereof. The semi-transparentcathode is of particular advantage if it is desired simultaneously toprovide an electron beam image of a pair of optical images directed atthe photocathode in opposite directions. Such a configuration isillustrated in FIG. 3, wherein semitransparent cathode 31 is substitutedfor opaque cathode 12 of FIG. 1.

In FIG. 3, semi-transparent photocathode 31 is simultaneously irradiatedby first and second optical images projected onto the photocathodeplanar electron emitting surface 32 via aligned optical axes 33 and 34.The optical images on axes 33 and 34 are respectively focused onelectron emitting surface 32 by lenses 35 and 36, with the latter imagebeing projected through photocathode 31 to surface 32. Electron emittingsurface 32 responds to both optical images simultaneously, wherebyelectrons indicative of the sum of the two impinging images propagatefrom photocathode 31 to the target planar receiving surface 10 that isparallel to electron emitting surface 32. The image on target 13 is,therefore, a composite of the images focused by lenses 35 and 36 on theelectron emitting surface 32 of photocathode 31. In the alternative,only one of the optical images can be focused at a time on cathodesurface 32.

While there have been described and illustrated several specificembodiments of the invention, it will be clear that variations in thedetails of the embodiments specifically illustrated and described may bemade without departing from the true spirit and scope of the inventionas defined in the appended claims. For example, principles of theinvention can be used with X-ray image converters and the term opticalis to be broadly interpreted to include X-rays and other similarparticulate matter, as well as energy from infrared to ultraviolet.

What is claimed is:

l. A device for converting an optical image into an electron beam imagecomprising a photocathode having a surface responsive to the opticalimage for deriving an electron beam in response to the optical imageimpinging thereon, a target electrode having a surface for receiving theelectron beam, said target electrode being displaced relative to thephotocathode so that it does not obstruct the optical image, a systemfor focusing the electron beam onto the receiving surface of the targetelectrode, said system including: means for providing a constant,homogeneous d.c. electric field throughout a region between saidsurfaces of the photocathode and electrode, and means for providing aconstant, homogeneous d.c. magnetic field throughout said region, saidfields having longitudinal vectors canted relative to each other so thatthe magnetic field longitudinal vector extends along an axis betweensaid surfaces.

2. The device of claim 1 wherein both said surfaces are planar andparallel to each other.

3. The device of claim 2 wherein said photocathode is opaque.

4. The device of claim 2 wherein said photocathode is semi-transparent.

5. The device of claim 2 wherein said photocathode is semi-transparent,and including means for directing a first optical image directly on saidplanar surface of said photocathode, and means for directing a secondoptical image on said planar surface of said photocathode viatransmission through the photocathode.

6. A device for converting an optical image into an electron imagecomprising a photocathode having a planar surface responsive to theoptical image for deriving an electron beam in response to the imageimpinging thereon, a target electrode having a planar surface forreceiving the electron beam, said target electrode being displacedrelative to the photocathode so that it does not obstruct the opticalimage, said planar surfaces being parallel to each other, whereby aregion exists between said planar surfaces that includes a longitudinalaxis at right angles to said planar surfaces and a further axis thatextends between said planar surfaces and is canted relative to thelongitudinal axis, and electron lens means for providing for electronsof said beam throughout said region: (a) constant acceleration along andparallel to the further axis, (b) cyclotron motion about the furtheraxis, and (c) constant radial velocity in planes perpendicular to theplane containing both said axes and parallel to the planar surfaces.

7. The device of claim 6 wherein said electron lens includes: means forproviding a constant and homogeneous d.c. electric field throughout saidregion along the longitudinal axis, and means for providing a constantand homogeneous d.c. magnetic field throughout said region along thefurther axis.

1. A device for converting an optical image into an electron beam imagecomprising a photocathode having a surface responsive to the opticalimage for deriving an electron beam in response to the optical imageimpinging thereon, a target electrode having a surface for receiving theelectron beam, said target electrode being displaced relative to thephotocathode so that it does not obstruct the optical image, a systemfor focusing the electron beam onto the receiving surface of the targetelectrode, said system including: means for providing a constant,homogeneous d.c. electric field throughout a region between saidsurfaces of the photocathode and electrode, and means for providing aconstant, homogeneous d.c. magnetic field throughout said region, saidfields having longitudinal vectors canted relative to each other so thatthe magnetic field longitudinal vector extends along an axis betweensaid surfaces.
 2. The device of claim 1 wherein both said surfaces areplanar and parallel to each other.
 3. The device of claim 2 wherein saidphotocathode is opaque.
 4. The device of claim 2 wherein saidphotocathode is semi-transparent.
 5. The device of claim 2 wherein saidphotocathode is semi-transparent, and including means for directing afirst optical image directly on said planar surface of saidphotocathode, and means for directing a second optical image on saidplanar surface of said photocathode via transmission through thephotocathode.
 6. A device for converting an optical image into anelectron image comprising a photocathode having a planar surfaceresponsive to the optical image for deriving an electron beam inresponse to the image impinging thereon, a target electrode having aplanar surface for receiving the electron beam, said target electrodebeing displaced relative to the photocathode so that it does notobstruct the optical image, said planar surfaces being parallel to eachother, whereby a region exists between said planar surfaces thatincludes a longitudinal axis at right angles to said planar surfaces anda further axis that extends between said planar surfaces and is cantedrelative to the longitudinal axis, and electron lens means for providingfor electrons of said beam throughout said region: (a) constantacceleration along and parallel to the further axis, (b) cyclotronmotion about the further axis, and (c) constant radial velocity inplanes perpendicular to the plane containing both said axes and parallelto the planar surfaces.
 7. The device of claim 6 wherein said electronlens includes: means for providing a constant and homogeneous d.c.electric field throughout said region along the longitudinal axis, andmeans for providing a constant and homogeneous d.c. magnetic fieldthroughout said region along the further axis.